Testing the Hypothesis

Pounds (1981) radio tracked both stoats and common weasels in the farmlands and sand dunes near Aberdeen, Scotland (see Table 8.1). He found that both preferred to hunt in the same habitats, the field margins and rough grasslands where small mammals were most abundant. Both stoats and common weasels could be found in such places at any time. Overall, there was a constant ratio of five stoats to 10 common weasels over an area of 54 km2, which implies, since stoats are twice the size of common weasels, that both species were making roughly equal demands on the local prey resources. Competition seemed to be unavoidable, especially in spring when the numbers of small mammals were at their annual low after 7 months (October to April) of unreplaced losses.

Pounds watched both species at a distance (30 to 50 m) with strong binoculars, using infrared lights after dark, and followed them on hunting expeditions. Both hunted at any time of day or night. The diameters of the burrows of the local field voles averaged 23 mm, and Pounds estimated that female common weasels could get into any but the narrowest ones. Males could get into the larger ones with a squeeze, but stoats of both sexes were excluded from all burrows.

By scat analysis, Pounds showed that field voles were the most important prey of common weasels all the year round, and also of stoats in autumn; for the rest of the year stoats concentrated on rabbits. There were differences in emphasis but, still, the overlap in their diets was substantial, especially between male weasels and female stoats. Members of the two species had no obvious way to partition their common prey resources, either in space or time. Pounds reckoned that competition for food between the two must be serious, and that no other consideration (e.g., shortage of den sites, predation by human or other large predators) was anywhere near as significant to them. Nevertheless, he concluded that the exploitation advantage of the common weasels' ability to hunt in tunnels was, most of the time, sufficient to ensure their survival and that the stoats' ability to hunt alternative prey over a larger area was sufficient to compensate for their restricted access to rodents.

The other side of the hypothesis suggests that stoats should be able to evict common weasels from choice hunting areas. In fact, of course, no open aggression is needed. The parties need not even meet face to face. The effect is the same so long as a common weasel always knows when a stoat is about and is scared enough of meeting it to move elsewhere rather than risk an encounter. Pounds noted that both species routinely marked their home ranges with scent signals, and these could be quite enough on their own to have the required effect, perhaps reinforced occasionally by an actual meeting.

Pounds carefully refrained from speculating any further, but he reported that, in the 1,300 hours of radio tracking he logged, he recorded no direct encounters between free-living stoats and common weasels. His experiments with captive animals in enclosures and in cages in the field strongly suggested that common weasels certainly could detect the presence of a stoat, and avoided it if they could. He reckoned the effect in the wild would be very limited in time and extent. Female weasels, which might be considered most vulnerable to interference from stoats, were actually least concerned by them, because they could always escape to a rodent tunnel.

Erlinge and Sandell (1988) made much use of enclosures, in which animals cannot be guaranteed to behave naturally, yet their observations complement Pounds' fieldwork in two of three respects. First, they showed that common weasels really are scared to death of face-to-face encounters with stoats. When one of each was released into a 30-m2 enclosure, the common weasel always fled to a refuge box, while the stoat took over the open area, moving about confidently and ignoring the weasel with lordly disdain. The weasel remained hidden but alert, watching the stoat and ready to react with threats if ever the stoat approached its refuge. If the stoat settled down somewhere quietly, the weasel cautiously emerged and began to make desperate attempts to escape.

Erlinge and Sandell also showed that common weasels tended to avoid traps that had previously been occupied by stoats and still reeked of stoat scent, whereas stoats treated weasel-scented traps the same as any others.

Finally, Erlinge and Sandell looked for field evidence to support their observations in the enclosure. They searched back through years of trapping records (1973 to 1984), checking for any evidence of reciprocal distributions of the two species in their study area. The two species were considered to be in potential contact if one was caught within 200 m of a site, and within 2 months before or after the date on which the other had been caught. In habitats that were potentially suitable for both, they recorded 21 weasels caught in places without stoats, but only six in places with them. Erlinge and Sandell concluded that common weasels avoid areas occupied by stoats, although they use them when the stoats are absent.

This result disagreed quite specifically with Pounds' radio-tracking observations. Consequently, King (unpublished) decided to repeat the test for reciprocal distribution, using the detailed trapping records kept by three gamekeepers at North Farm, Sussex, in 1974 and 1975, and kindly lent by the Game Conservancy. The keepers had a network of about 300 permanent Fenn steel trap sites spread evenly over about 1,500 ha, mostly along field boundaries and rough vehicle tracks, as mapped by Tapper (1982). These were kill traps, so each individual was removed as soon as it was caught, but the scent marks left by residents continued to proclaim home range boundaries for some time after the owners were gone.

The entire area, except in the centers of open fields, was suitable habitat for both species. The first step was to mark the trap sites where stoats were caught, then to mark all captures of common weasels, distinguishing those that could have been in contact with a stoat by Erlinge and Sandell's definition. The common weasels clearly did not avoid the areas occupied by stoats; over a quarter of them (64 of 246) were potentially "in contact," and 15 were caught in the same trap as a stoat within 30 days.

This is, of course, a very rough method of estimating the relative distributions of the two species over such a large area, and even if the results had shown the mutual avoidance claimed, there would be no way to tell why. "Unfavored" areas could be unoccupied by common weasels not only because stoats were there but also because hunting conditions were better elsewhere, or simply because the density of common weasels was low in that year.

The only reliable method of tackling the problem is the way Pounds did it, by direct observation, and his conclusion was that, if the habitat is favorable to both, both will use it. There may well be mutual avoidance, but it must be on a rather fine scale under normal conditions, and becomes a critical factor only in places (such as on small islands) where food is severely limited and confrontations inescapable.

Recent studies have documented in great detail the differences in habitat selection, activity, and diet between sympatric populations of two weasel species (McDonald 2002; Aunapuu & Oksanen 2003; St.-Pierre et al. in press-a; Sidorovich et al. unpublished). The larger species usually chooses larger prey and the most productive habitats, capable of supplying their higher demands for food; the smaller species is more affected by changes in the abundance of small rodents, because it has fewer alternatives. But the question of whether the larger one can actively exclude the smaller, or whether the smaller simply avoids the larger, remains very difficult to answer. As Aunapuu and Oksanen (2003) put it, "The mechanisms of competitive coexistence cannot be derived from the preferences or from the food habits in times of prey abundance. The theory of exploitation competition .. . does not deal with preferences but with the ability of organisms to persist in times of shortage." Weasels are very scarce at such times, so the chances of getting field data on their interactions then depend on the development of new technology for field observation.

Relationships among the North American species might be different, or at least more complicated, because both stoats and longtails can be the large weasel of a pair. Stoats may be either the larger weasel, when stoats are sympatric with least weasels, or the small weasel, when sympatric with longtails. Inside a narrow zone across North America (Chapter 1), all three weasels may be found, although they seldom all live in the same place at the same time. Thus, the scenario of conditional coexistence, followed by local extinction of one or both (or all three!) and later recolonization, seems to be generally true. Indeed, several researchers have reported locally reciprocal distributions among sympatric weasels, such as stoats moving out when longtails arrived (Fitzgerald 1977; Simms 1979b; Gamble 1980).

The distribution of North American weasels has one curious feature. Stoats and least or common weasels are found together from the Arctic right down to around 40°N on both sides of the Atlantic, but longtails live only south of about 55°N. If longtails are capable of displacing the smaller species by aggressive interference, why do they not extend into the forests and grasslands further north?

The answer may lie in a thoughtful hypothesis by Simms (1979a), which he suggested quite independently but has intriguing similarities with the one discussed above. Simms noted that the small stoats of the snowy north feed primarily on voles. They are well adapted to hunt in rodent tunnels and are strongly specialized as underground or under-snow predators. Long-tailed weasels are larger and have more general food habits. Simms suggested that longtails are confined to the south by the prolonged snow cover of the north, because they are less capable of hunting through the confined under-snow spaces than smaller weasels, and because the only alternative in the far north is to compete with the larger predators such as foxes and martens, which hunt above the snow. Conversely, stoats are confined to the north by interference competition from longtails.

Longtails can and do, in fact, hunt under the snow when it is deep enough and when coarse woody debris and sugar snow allow access to subnivean spaces (see Figure 6.2). From the Upper Midwest and across the Rockies to the western, coastal mountains, longtails and American martens, their larger cousins, spend a lot of time under the snow (Francis & Stephenson 1972; Fitzgerald 1977; Buskirk et al. 1989; Buskirk & Powell 1994). The key to the longtail's success is its ability to switch to alternative prey when rodents are scarce, but boreal forests have few prey for weasels other than small rodents. Perhaps it is this poor diversity of prey that keeps longtails mainly in the south, regardless of snow or other weasels (Fitzgerald 1977; Gamble 1981). If so, we are left with another question: If boreal forests lack diverse prey, how can stoats and least weasels coexist there?

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